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Sleep: The Clean-Up Crew of a Dirty Mind

When you throw a party, you don’t clean up until everyone’s gone home.

Sleep is a VERY active state, not passive at all.

The brain parties every moment you are awake. While it’s being lively it makes a mess, like partiers everywhere. All living cells metabolize energy. As they burn fuel they leave behind residue and toxic wastes—the equivalent of empty glasses, smelly ashtrays, and dirty dishes that a host must face when the action is over and things have died down.

To tidy up the place a host must exert energy to reverse the disarray (its entropy, to use the scientific name for disorder). But how the brain flushes out waste has been a mystery until now. That’s because, unlike every other organ in the body, the brain has no lymphatic channels, the separate drainage highway that transports white blood cells, antigens, and metabolic waste to the lymph nodes.

In humans, 700 such lymph nodes filter the liquid before returning it to the circulatory system by way of the big subclavian veins in the neck. Blood circulates in a loop, but lymph flows only in one direction towards the heart. Other parts of the lymphatic system include the tonsils, adenoids, spleen, and thymus.

How does the brain get around the shortcoming of having no lymphatics? The answer, published in the journal Science, may lie in the cerebrospinal fluid, or CSF. This brilliant clear fluid is constantly secreted into the nervous system, and subsequently reabsorbed. CSF is manufactured inside the hollow ventricles. The fluid circulates over the outer brain surface and spinal cord, and then percolates through nervous tissue and bathes the entire “interstitial” space between individual cells. The fluid gets reabsorbed into veins, enters the general blood circulation, and the cycle begins anew.

The ventricles, where CSF is manufactured.

During this unique rinsing cycle it appears that the brain may not only flush away toxic waste but also pick up beneficial molecules and nutrients. At the moment, however, we know most about how it handles waste. And what we do know is pretty surprising because it contradicts what we’ve thought was the case for decades.

Sleep deprivation builds up the concentrations of both beta–amyloid and neural plaques, two hallmark abnormalities of Alzheimer’s disease. For decades we have known that demented individuals do not sleep well. We have traditionally attributed that to the degeneration their brains were undergoing. But researchers have begun to question whether insomnia might actually be a cause of mental decline rather than its result.

In mice, where direct observation of brain tissue is possible, CSF that is dyed with a visible marker circulates briskly when animals are anesthetized or naturally asleep, but not when they are awake. Toxins, including beta-amyloid, are flushed away twice as rapidly in sleeping brains than in wakeful ones.

CSF flows faster during sleep (left) than waking (right).

A follow-on study in humans confirmed the finding by recording rising levels of beta-amyloid during wakefulness and falling ones during sleep. The pattern is particularly robust in teenagers and young adults. So mothers, as usual, are right: young people need their sleep if they want to be mentally sharp the next day.

What the long-term effects of sleep deprivation are with respect to dementia are not yet clear. For now it remains a chicken-and-egg question. But it’s something you may want to sleep on.